Shimadzu EuropaDrinking water analysis using a simultaneous ICP Spectrometer

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For more than 30 years, inductively coupled plasma (ICP) has been used as an excitation source for optical emission spectrometers and has become an indispensable tool for monitoring of drinking water.

As water is the basis for a healthy life, strict control as well as sophisticated and reliable water purification and supply are crucial conditions for health and the prevention of epidemics all over the world. The risk of contamination is high as more than 100,000 chemical substances can contaminate our drinking water every day.

That´s why drinking water is regarded as the best controlled food sample on the global scale and is therefore continuously monitored according to the European 98/83/EG directive and in the focus of the WHO guidelines for drinking water quality.

Inductively coupled plasma optical emission spectrometry (ICP-OES) is widely used for accurate determination of major, minor, and trace elements in aqueous solutions. Experimental data has been generated using international reference material as well as real life samples from various locations. Performance of different system configurations has been evaluated, and optimized methods have been prepared for achieving highest sensitivity and lowest detection limits. Multielement analysis of aqueous solutions in compliance with international drinking water regulations is one of the main application areas of ICP-OES.

Experimental Conditions

The system used in this study was the Shimadzu ICPE- 9820, a so called dual view ICP-OES spectrometer which allows axial and radial plasma observation in the same analytical run. Its high-performance echelle optics with special “Schmidt mirror” enable the effective use of the entire 1024 × 1024 pixel CCD detector area. In this way, a resolution of higher than 0.005 nm is attained over the entire wavelength range of 167–800 nm. The detector, which has an antiblooming function, reliably acquires signal intensities, even at long exposure times. All samples can therefore be determined accurately within one single analysis sequence, including samples with very different element concentrations. The “reprocessing” function of the ICPEsolution software enables the determination of additional elements or changing of concentration range for alternate wavelengths without the need for new measurements. The vacuum optics combined with mini torch technology reduce argon gas consumption considerably. The torch used here reduces argon gas consumption by half that of conventional torches, without loss in sensitivity. In addition, time-consuming rinsing of the optics with ultrapure gas is no longer necessary. The system is ready for operation and stable within the shortest possible time. Equipped with the optional autosampler, the system can be fully automated for high sample throughput operation. The system status is monitored continuously and can be retrieved at any time, whenever needed.

Easy Method development

The software described earlier offers users additional support with two integrated assistant functions. The “Development Assistant” creates complex calibrations, from the selection of optimal wavelengths up to the composition of standard concentrations. In combination with the “Monitor Function” for qualitative analysis, this assistant points to possible interference problems or incorrect wavelength selection before the actual calibration takes place, and displays solutions for method modification or error correction. The “Diagnosis Assistant” evaluates data already measured, and compares this with information from various databases. Data evaluation and recalculation have never been more straightforward, as the complete emission spectrum of a sample is continuously available.

What is inside the drinking water?

Water is a natural product, and contains many substances such as organic compounds or inorganic constituents. The term mineral water already points to some of these substances – minerals such as calcium, potassium, magnesium and sodium. These inorganic nutrients are essential as the human body does not synthesize them, and they must therefore be obtained from a dietary source. There are, however, many other essential elements in drinking water such as the trace elements chromium, cobalt, iron, copper, manganese, selenium and zinc. Other tentative candidates, whose exact functions as trace elements in the human body have not yet been conclusively investigated, could for instance be arsenic, nickel or tin. For all of these elements, the concentration plays a key role. For example, lack of selenium leads to a deficiency, and selenium-dependent enzymes that are present in almost every organ cannot perform their function. However, too much selenium can lead to poisoning, so-called selenosis that includes symptoms ranging from fatigue and nausea to hair and nail loss. On the topic of drinking water analysis, it is important that food monitoring can rule out an overdose as a cause for poisoning. Drinking water, for instance, may contain a maximum of 10 µg/L selenium. Other elements in the limit value list of the 98/83/EC directive are heavy metals such as lead, cadmium, chromium, cobalt, copper, manganese, molybdenum, nickel, mercury, zinc and tin. It becomes apparent from this listing that the classification of the elements overlaps. Some of the heavy metals were classified as ‘essential’. Other elements, on the other hand, can be classified as toxic or even belong to both categories. This emphasizes once more that the concentration is decisive.

Drinking water analysis

The elements specified in the 98/83/EC directive should be analyzed with a minimum of effort and within the shortest possible time. These elements, along with other important minerals, are listed in Table 1.

Table 1. The elements and their limit values specified in 98/83/EC, as well as the required detection limits of the analytical instruments. The detection limits specified for the ICPE-9820 refer to the present application.

[1] In addition to the 70 elements that can be determined using the ICPE-9800 series, other elements can be included in the analysis.

[2] Determined as 3-fold standard deviation of a natural sample with a low concentration of the element. The detection limits refer to the drinking water application and can be improved depending on the selection of the spectral line/application.

[3] The limit value in Germany according to the German Drinking Water Ordinance is 3,0 µg/L, the detection limit is 0.3 µg/L.

[*] using the hydride system.

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When a large number of elements have to be determined, the ICPE-9800 series proves to be particularly advantageous because it can determine all elements simultaneously. Of this series, the ICPE-9810 is suitable for the determination of ultra-trace concentrations of most of the elements mentioned. The ICPE-9810 is operated under axial plasma observation (AX). Since higher ppm-ranges, such as for sodium, are also of interest, radial plasma observation (RD) is required as well. The ICPE-9820 with “dual view” offers this combination of axial and radial plasma observation.

The exact effect of axial and radial plasma observation is shown in the example for sodium. The calibration series up to 200 mg/L is observed both axially (complete plasma) and radially (plasma section, from the side). The calibration curve under axial observation is not linear, as so-called ionization interferences can occur, especially at high concentrations of the alkaline elements sodium and potassium. These can be masked using radial plasma observation.

The detection limits required according to the European 98/83/EC directive can be attained using the ICPE-9820. Figure 2 shows all calibrations curves. Certified reference materials – drinking water samples with known content of the elements listed in Table 1 – were also measured as unknown samples within one measuring sequence. Three different reference material samples (TMDW, trace metals in drinking water), supplied by High Purity Standards (North Charleston, SC, USA), were investigated.

Figure 2. Radial observation masks ionization interferences at high sodium concentrations (right) and the linear working range can be markedly extended.

Table 2. Recoveries of the elements that are contained in the reference material lie within the range of 100 ± 5 %.

[1] The certified reference values are reported with an uncertainty of 0.5 - 0.2. %.

[2] n.r. = not reported.

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The results in Table 2 show that the certified concentrations were recovered within a short time and with very little effort. The elements mercury, arsenic, antimony and selenium can be measured sensitively when switching to the hydride system.

Conclusion

The ICPE-9820 series allows easy handling and highest sample throughput. The system complies with the latest standards according to the European 98/83/EC directive and can also be used for many other samples in the field of food analysis such as beer, wine and other beverages. For more information please follow: www.shimadzu.eu/food-beverages-agriculture.